In pre-historic times I (EDIT: designed and) built a battery powered heart rate data logger for livestock. I used an instrumentation amplifier IC and some very simple electrodes and was able to use auto-correlation to detect the heart rate pretty reliably. For humans today I'd pick something like a Garmin chest strap which works with Bluetooth or ANT+. I've no direct experience with ANT or ANT+ but I imagine it's not hard to interface with ;)

That's more or less what I was gonna say...